Polymeric materials are commonly used in the manufacture of modules comprising one or more electronic devices including, but not limited to, solar cells (also known as photovoltaic cells) (or PV cells), liquid crystal panels, electro-luminescent devices and plasma display units. The modules often comprise an electronic device in combination with one or more substrates, e.g., one or more glass cover sheets, often positioned between two substrates in which one or both of the substrates comprise glass, metal, plastic, rubber or another material. The polymeric materials are typically used as the encapsulant or sealant for the module or, depending upon the design of the module, as a skin layer component of the module, e.g., a backskin in a solar cell module. Typical polymeric materials for these purposes include silicone resins, epoxy resins, polyvinyl butyral resins, cellulose acetate, ethylene-vinyl acetate copolymer (EVA) and ionomers.
A multi-layer polymeric sheet is frequently used as the backsheet (or backskin, substrate) for solar cell (photovoltaic) modules. The sheet can comprise a first polymer layer comprising a film of polyvinyl fluoride (PVF) or polyvinylidene fluoride (PVDF) having an adhesive coating on one side, a second optionally pigmented polymer layer extruded on the adhesive coating of the first polymer layer; and, optimally, a third polymer layer for providing the moisture barrier and dielectric insulation. The sheet is required to have good adhesion strength to the encapsulant, mechanical strength, electrical insulation, and moisture blocking to protect the solar cells and metallic components in modules from environmental elements. Further, the backsheet must be electrically insulating to prevent a conduction path from the back contacts of the device to the grounded metal frame. The backsheet material must also prevent moisture ingress because this is the generally accepted mode of failure for devices that do not pass the IEEE 1262 qualification test. IEEE 1262 is commonly referred to as the “damp heat test” and for part of the test PV devices are subjected to 85° C. and 85% relative humidity (RH) for 1000 hours as per the specification. Thus, good polymeric backsheet materials are required to have good adhesion with the encapsulant, good electric insulation, mechanical strength, and moisture blocking to protect the solar cell and metallic components in modules from environmental elements.
Current backsheets require more than one processing step to make. Frequently, the individual film layers are each made separately and then laminated together, often with intermediate adhesive layers. For example, U.S. Pat. No. 6,521,825 B2 discloses a solar cell module comprising a three-layer laminated film. Two of the layers are heat and weather resistant while the core layer is moisture resistant.
Currently, several multilayer films are used as the backsheet for PV module, such as TEDLAR®/PET/TEDLAR®, TEDLAR®/PET/EVA, and TEDLAR®/Aluminum foil/TEDLAR®. TEDLAR® is a registered trademark of DuPont for polyvinyl fluoride, PET is polyethylene terephthalate, and EVA is ethylene vinyl acetate. These multi-layer materials provide adequate mechanical strength and UV stability, but the moisture barrier property is not good enough for solar cell long term durability. Aluminum foil has been used as the central layer to provide moisture barrier properties, but the industry has concerns that putting a conductive layer inside the backsheet might compromise the long-term dielectric property of the backsheet.
EVA copolymers with a high content (28 to 35 wt %) of units derived from the vinyl acetate monomer are commonly used to make encapsulant film for use in photovoltaic (PV) modules. See, for example, WO 95/22844, 99/04971, 99/05206 and 2004/055908. EVA resins are typically stabilized with ultra-violet (UV) light additives, and they are typically crosslinked during the solar cell lamination process using peroxides to improve heat and creep resistance to a temperature between about 80° C. and 90° C. However, EVA resins are less than ideal PV cell encapsulating film material for several reasons. For example, EVA film progressively darkens in intense sunlight due to the EVA resin chemically degrading under the influence of UV light. This discoloration can result in a greater than 30% loss in power output of the solar module after as little as four years of exposure to the environment. EVA resins also absorb moisture and are subject to decomposition.
Recent development in thin film photovoltaic technologies, such as CuInGaSe2 (CIGS) polycrystalline thin film photo-voltaic cells, also require a better moisture barrier property from the PV module backsheet. Backsheets with an EVA layer used for a CIGS PV module have problems passing the “damp heat test” (85° C. and 85% relative humidity (RH) for 1000 hours as per the specification). Thus, a backsheet material that has good adhesion to the encapsulant, good electric insulation, mechanical strength, and moisture blocking is needed for the PV industry.